Amorphous oxide and thin film transistor

ABSTRACT

The present invention relates to an amorphous oxide and a thin film transistor using the amorphous oxide. In particular, the present invention provides an amorphous oxide having an electron carrier concentration less than 10 18 /cm 3 , and a thin film transistor using such an amorphous oxide. In a thin film transistor having a source electrode  6 , a drain electrode  5 , a gate electrode  4 , a gate insulating film  3 , and a channel layer  2 , an amorphous oxide having an electron carrier concentration less than 10 18 /cm 3  is used in the channel layer  2.

CROSS-REFERENCE TO RELATED APPLICATIONS

This is a continuation application of U.S. patent application Ser. No.12/504,158, filed on Jul. 16, 2009, which is divisional of U.S. patentapplication Ser. No. 10/592,431, filed on Sep. 11, 2006, now abandoned,which is a 371 of International Application No. PCT/JP05/03273, filed onFeb. 28, 2005, which claims the benefit of priority from the priorJapanese Patent Application Nos. 2004-071477, filed on Mar. 12, 2004 and2004-325938 filed on Nov. 10, 2004, the entire contents of which areincorporated herein by references.

TECHNICAL FIELD

The present invention relates to amorphous oxides and thin filmtransistors.

BACKGROUND ART

A thin film transistor (TFT) is a three-terminal element having a gateterminal, a source terminal, and a drain terminal. It is an activeelement in which a semiconductor thin film deposited on a substrate isused as a channel layer for transportation of electrons or holes and avoltage is applied to the gate terminal to control the current flowingin the channel layer and switch the current between the source terminaland the drain terminal. Currently, the most widely used TFTs aremetal-insulator-semiconductor field effect transistors (MIS-FETs) inwhich the channel layer is composed of a polysilicon or amorphoussilicon film.

Recently, development of TFTs in which ZnO-based transparent conductiveoxide polycrystalline thin films are used as the channel layers has beenactively pursued (Patent Document 1). These thin films can be formed atlow temperatures and is transparent in visible light; thus, flexible,transparent TFTs can be formed on substrates such as plastic boards andfilms.

However, known ZnO rarely forms a stable amorphous phase at roomtemperature and mostly exhibits polycrystalline phase; therefore, theelectron mobility cannot be increased because of the diffusion at theinterfaces of polycrystalline grains. Moreover, ZnO tends to containoxygen defects and a large number of carrier electrons, and it is thusdifficult to decrease the electrical conductivity. Therefore, it hasbeen difficult to increase the on/off ratio of the transistors.

Patent Document 2 discloses an amorphous oxide represented byZn_(x)M_(y)In_(z)O_((x+3y/2+3z/2)) (wherein M is at least one elementselected from Al and Ga, the ratio x/y is in the range of 0.2 to 12, andthe ratio z/y is in the range of 0.4 to 1.4). However, the electroncarrier concentration of the amorphous oxide film obtained herein is10¹⁸/cm³ or more. Although this is sufficient for regular transparentelectrodes, the film cannot be easily applied to a channel layer of aTFT. This is because it has been found that a TFT having a channel layercomposed of this amorphous oxide film does not exhibit a sufficienton/off ratio and is thus unsuitable for TFT of a normally off type.

-   Patent Document 1: Japanese Unexamined Patent Application    Publication No. 2003-298062-   Patent Document 2: Japanese Unexamined Patent Application    Publication No. 2000-044236

DISCLOSURE OF INVENTION

An object of the present invention is to provide an amorphous oxidehaving a low electron carrier concentration and to provide a thin filmtransistor having a channel layer composed of such an amorphous oxide.

The present invention provides: (1) an amorphous oxide having anelectron carrier concentration less than 10¹⁸/cm³. In the presentinvention, the electron carrier concentration of the amorphous oxide ispreferably 10¹⁷/cm³ or less or 10¹⁶/cm³ or less.

The present invention also provides: (2) an amorphous oxide in whichelectron mobility thereof increases with the electron carrierconcentration.

The present invention also provides: (3) the amorphous oxide accordingto item (1) or (2) above, in which the electron mobility is more than0.1 cm²/(V·sec).

The present invention also provides: (4) the amorphous oxide accordingto item (2) or (3) above, exhibiting degenerate conduction. Note that“degenerate conduction” used herein is defined as a state in which thethermal activation energy for temperature dependency of electricalresistance is 30 meV or less.

Another aspect of the present invention provides: (5) the amorphousoxide according to any one of items (1) to (4) above, in which theamorphous oxide is a compound that contains at least one elementselected from Zn, In, and Sn as a constituent and is represented by[(Sn_(1-x)M4_(x))O₂]a.[In_(1-y)M3_(y))₂O₃]b.[(Zn_(1-z)M2_(z))O]c(wherein 0≦x≦1, 0≦y≦1, 0≦z≦1; x, y, and z are not simultaneously 1;0≦a≦1, 0≦b≦1, 0≦c≦1, and a+b+c=1; M4 is a group IV element (Si, Ge, orZr) having an atomic number smaller than that of Sn; M3 is Lu or a groupIII element (B, Al, Ga, or Y) having an atomic number smaller than thatof In; and M2 is a group II element (Mg or Ca) having an atomic numbersmaller than that of Zn).

In the present invention, the amorphous oxide according (5) above mayfurther contain at least one element selected from group V elements (V,Nb, and Ta) M5 and W.

Another aspect of the present invention provides: (6) a thin filmtransistor including the amorphous oxide according to any one of (1) to(4) above, in which the amorphous oxide is a single compound representedby [(In_(1-y)M3_(y))₂O₃(Zn_(1-x)M2_(x))O]_(m) (wherein 0≦x≦1; 0≦y≦1; xand y are not simultaneously 1; m is zero or a natural number less than6; M3 is Lu or a group III element (B, Al, Ga, or Y) having an atomicnumber smaller than that of In; and M2 (Mg or Ca) is a group II elementhaving an atomic number smaller than that of Zn) in a crystallized stateor a mixture of the compounds with different values of m. M3 is, forexample, Ga, and M2 is, for example Mg.

The present invention also provides the amorphous oxide according to anyone of (1) to (6) above formed on a glass substrate, a metal substrate,a plastic substrate, or a plastic film. The present invention alsoprovides a field effect transistor including a channel layer composed ofthe amorphous oxide described above. The field effect transistor of thepresent invention is characterized in that the gate insulating film isone of Al₂O₃, Y₂O₃, and HfO₂ or a mixed crystal compound containing atleast two of these compounds.

Another aspect of the present invention provides: (7) a transparentsemi-insulating amorphous oxide thin film comprising In—Ga—Zn—O, inwhich the composition in a crystallized state is represented byInGaO₃(ZnO)_(m) (wherein m is a number less than 6 and 0<x≦1), theelectron mobility is more than 1 cm²/(V·sec) and the electron carrierconcentration is less than 10¹⁸/cm³.

Furthermore, the present invention also provides: (8) a transparentsemi-insulating amorphous oxide thin film comprising In—Ga—Zn—Mg—O, inwhich the composition in a crystallized state is represented byInGaO₃(Zn_(1-x)Mg_(x)O)_(m) (wherein m is a number less than 6 and0<x≦1), the electron mobility is more than 1 cm²/(V·sec) and theelectron carrier concentration is less than 10¹⁸/cm³. Moreover, thepresent invention also provides a method for forming the transparentsemi-insulating amorphous oxide thin film in which an impurity ion forincreasing the electrical resistance is not intentionally added and thedeposition is conducted in an atmosphere containing oxygen gas.

A thin-film transistor according to another aspect of the presentinvention includes a source electrode, a drain electrode, a gateelectrode a gate insulating film and a channel layer, in which thechannel layer contains an amorphous oxide having an electron carrierconcentration of less than 10¹⁸/cm³. Preferably, the electron carrierconcentration of the amorphous oxide is 10¹⁷/cm³ or less or 10¹⁶/cm³ orless. The amorphous oxide is an oxide containing In, Ga, and Zn, inwhich the atomic ratio In:Ga:Zn is 1:1:m (m<6). Alternatively, theamorphous oxide is an oxide including In, Ga, Zn, and Mg, in which theatomic ratio In:Ga:Z_(1-x)Mg_(x) is 1:1:m (m<6), wherein 0<x≦1.

The amorphous oxide is selected from In_(x)Ga_(1-x) oxides (0≦x≦1),In_(x)Zn_(1-x) oxides (0.2≦x≦1), In_(x)Sn_(1-x) oxides (0.8≦x≦1), andIn_(x)(Zn, Sn)_(1-x) oxides (0.15≦x≦1).

In a thin film transistor of the present invention, a material in whichthe electron mobility increases with the electron carrier concentrationcan be used as the amorphous oxide.

According to the present invention, an amorphous oxide having a lowelectron carrier concentration can be provided, and a thin filmtransistor including a channel layer composed of such an amorphous oxidecan be provided.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph that shows the relationship between the oxygen partialpressure during the deposition and the electron carrier concentration ofan In—Ga—Zn—O amorphous oxide deposited by a pulsed laser depositionmethod.

FIG. 2 is a graph that shows the relationship between the electroncarrier concentration and electron mobility of an In—Ga—Zn—O amorphousoxide film formed by a pulsed laser deposition method.

FIG. 3 is a graph that shows the relationship between the oxygen partialpressure during the deposition and the electrical conductivity of anIn—Ga—Zn—O amorphous oxide deposited by a high-frequency sputteringmethod.

FIG. 4 is a graph showing changes in electron conductivity, electroncarrier concentration, and electron mobility of InGaO₃(Zn_(1-x)Mg_(x)O)₄deposited by pulsed laser deposition against x.

FIG. 5 is a schematic illustration showing a structure of a top gate TFTelement.

FIG. 6 is a graph showing a current-voltage characteristic of a top gateTFT element.

FIG. 7 is a schematic illustration showing a pulsed layer depositiondevice.

FIG. 8 is a schematic illustration showing a sputter deposition device.

BEST MODE FOR CARRYING OUT THE INVENTION

An amorphous oxide of the present invention is characterized in that theelectron carrier concentration is less than 10¹⁸/cm³. A thin filmtransistor (TFT) of the present invention is characterized in that anamorphous oxide having an electron carrier concentration less than10¹⁸/cm³ is used in the channel layer.

For example, as shown in FIG. 5, the TFT is made by forming a channellayer 2 on a substrate 1 and a gate insulating film 3, a gate electrode4, a source electrode 6, and a drain electrode 5 on the channel layer 2.In this invention, an amorphous oxide having an electron carrierconcentration less than 10¹⁸/cm³ is used in the channel layer.

The structure of the TFT to which the present invention can be appliedis not limited to the staggered structure (top-gate structure) shown inFIG. 5 in which a gate insulating film and a gate terminal (electrode)are sequentially stacked on a semiconductor channel layer. For example,the TFT may have an inverted staggered structure (bottom-gate structure)in which a gate insulating film and a semiconductor channel layer aresequentially stacked on a gate terminal. The electron carrierconcentration mentioned above is a value measured at room temperature.Room temperature is, for example, 25° C. and, in particular, isappropriately selected from the range of about 0° C. to about 40° C.

The electron carrier concentration of the amorphous oxide of the presentinvention need not be less than 10¹⁸/cm³ all through the range of 0° C.to 40° C. For example, it is sufficient if the carrier electronconcentration is less than 10¹⁸/cm³ at 25° C. When the electron carrierconcentration is reduced to 10¹⁷/cm³ or less and more preferably to10¹⁶/cm³ or less, TFTs of a normally off type can be obtained in highyield. The electron carrier concentration can be determined byhall-effect measurement.

In the present invention, “amorphous oxide” is defined as an oxide thatshows a halo pattern in an X-ray diffraction spectrum and exhibits noparticular diffraction line. The lower limit of the electron carrierconcentration of the amorphous oxide of the present invention is notparticularly limited as long as the oxide can be used as the TFT channellayer. The lower limit is, for example, 10¹²/cm³.

Thus, in the present invention, the starting materials, compositionratio, production conditions, and the like of the amorphous oxide arecontrolled as in the individual examples described below so as to adjustthe electron carrier concentration to 10¹²/cm³ or more but less than10¹⁸/cm³. Preferably, the electron carrier concentration is adjusted to10¹³/cm³ to 10¹⁷/cm³, and more preferably 10¹⁵/cm³ to 10¹⁶/cm³.

The electron mobility is preferably 0.1 cm²/(V·sec) or more, morepreferably 1 cm²/(V·sec) or more, and most preferably 5 cm²/(V·sec) ormore when measured at room temperature. The amorphous oxide exhibitsincreased electron mobility as the electron carrier concentrationincreases. The conductivity thereof tends to exhibit degenerateconduction. Degenerate conduction is defined as a state in which thethermal activation energy for temperature dependency of electricalresistance is 30 meV or less.

(Starting Materials for Amorphous Oxide)

The amorphous oxide of the present invention contains at least oneelement selected from Zn, In, and Sn as a constituent component and isrepresented by[(Sn_(1-x)M4_(x))O₂]a.[(In_(1-y)M3_(y))₂O₃]b.[(Zn_(1-z)M2_(z))O]c[0≦x≦1, 0≦y≦1, 0≦z≦1; x, y, and z are not simultaneously 1; 0≦a≦1,0≦b≦1, 0≦c≦1, and a+b+c=1; M4 is a group IV element (Si, Ge, or Zr)having an atomic number smaller than that of Sn; M3 is Lu or a group IIIelement (B, Al, Ga, or Y) having an atomic number smaller than that ofIn and M2 is a group II element (Mg or Ca) having an atomic numbersmaller than that of Zn. The amorphous oxide may further contain atleast one element selected from group V elements M5 (V, Nb, and Ta) andW. In this description, the group II, III, IV, and V elements in theperiodic table are sometimes referred to as group 2, 3, 4, and 5elements, respectively; however, the meaning is the same.

The electron carrier concentration can be further decreased by adding atleast one element that can form a compound oxide, the at least oneelement being selected from a group 2 element M2 (M2: Mg or Ca) havingan atomic number smaller than that of Zn; Lu and a group 3 element M3(M3: B, Al, Ga, or Y) having an atomic number smaller than that of In; agroup 4 element M4 (M4: Si, Ge, or Zr) having an atomic number smallerthan that of Sn; and a group 5 element M5 (M5: V, Nb, and Ta) or W.

The elements M2, M3, and M4 having atomic numbers smaller than those ofZn, In, and Sn, respectively, have higher ionicity than Zn, In and Sn;thus, generation of oxygen defects is less frequent, and the electroncarrier concentration can be decreased. Although Lu has a larger atomicnumber than Ga, the ion radius is small and the ionicity is high,thereby achieving the same functions as those of M3. M5, which isionized at a valency of 5, strongly bonds to oxygen and rarely causesoxygen defects. Tungsten (W), which is ionized at a valency of 6,strongly bonds to oxygen and rarely causes oxygen defects.

The amorphous oxide applicable to the present invention is a singlecompound having a composition in a crystallized state represented by[(In_(1-y)M3_(y))₂O₃(Zn_(1-x)M2_(x))O]_(m) (wherein 0≦x≦1; 0≦y≦1; x andy are not simultaneously 1; m is zero or a number or a natural numberless than 6; M3 is Lu or a group 3 element (B, Al, Ga, or Y) having anatomic number smaller than that of In; and M2 is a group 2 element (Mgor Ca) having an atomic number smaller than that of Zn] or a mixture ofcompounds with different values of m. M3 is, for example, Ga. M2 is, forexample, Mg.

The amorphous oxide applicable to the present invention is a unitary,binary, or ternary compound within a triangle with apexes of SnO₂,In₂O₃, and ZnO. Among these three compounds, In₂O₃ has high amorphousformation capacity and can form a completely amorphous phase when In₂O₃is deposited by a vapor phase method while adding approximately 0.1 Paof water into the atmosphere.

ZnO and SnO₂ in some cases do not form an amorphous phase by themselves;however, they can form an amorphous phase in the presence of In₂O₃ as ahost oxide. In particular, of binary compositions containing two of theabove-described three compounds (compositions located on the side of thetriangle), the In—Zn—O system can form an amorphous film when In iscontained in an amount of about 20 at % or more, and the Sn—In—O systemcan form an amorphous film when In is contained in an amount of about 80at % or more by a vapor phase method.

In order to obtain an In—Zn—O amorphous film by a vapor phase method,about 0.1 Pa of steam may be introduced into the atmosphere. In order toobtain an In—Sn—O-system amorphous film by a vapor phase method, about0.1 Pa of nitrogen gas may be introduced into the atmosphere. For theternary composition, Sn—In—Zn, containing the three compounds, anamorphous film can be obtained by a vapor phase method when In iscontained in an amount of about 15 at % in the above-describedcomposition range. Note that “at %” herein indicates atomic percent withrespect to the metal ions other than oxygen ions. In particular, forexample, “the In—Zn—O system containing about 20 at % or more of In” isequivalent to In_(x)Zn_(1-x) (x>0.2).

The composition of the amorphous oxide film containing Sn, In, and/or Znmay contain additional elements as described below. In particular, atleast one element that forms a compound oxide, the at least one elementbeing selected from a group 2 element M2 (M2: Mg or Ca) having an atomicnumber smaller than that of Zn, Lu or a group 3 element M3 (M3: B, Al,Ga, or Y) having an atomic number smaller than that of In, and a group 4element M4 (M4: Si, Ge, or Zr) having an atomic number smaller than thatof Sn may be added. The amorphous oxide film of the present inventionmay further contain at least one element that can form a compound oxide,the at least one element being selected from group 5 elements (M5: V,Nb, and Ta) and W.

Addition of the above-described elements will increase the stability ofthe amorphous film and expands the composition range that can give anamorphous film. In particular, addition of highly covalent B, Si, or Geis effective for stabilization of the amorphous phase, and a compoundoxide composed of ions with largely different ion radii can stabilizethe amorphous phase. For example, in the In—Zn—O system, a stableamorphous film is rarely obtained at room temperature unless the rangeof In content is more than about 20 at %. However, by adding Mg in anequivalent amount to In, a stable amorphous film can be obtained at anIn content of more than about 15 at %.

An example of the amorphous oxide material that can be used in thechannel layer of the TFT of the present invention is described next. Theamorphous oxide that can be used in the channel layer is, for example,an oxide that contains In, Ga, and Zn at an atomic ratio satisfyingIn:Ga:Zn=1:1:m, wherein m is a value less than 6. The value of m may bea natural number but is not necessarily a natural number. This appliesto “m” referred to in other sections of this description. The atomicratio can be considered as equivalent to a molar ratio.

A transparent amorphous oxide thin film whose composition in acrystallized state is represented by InGaO₃(ZnO)_(m) (wherein m is anumber less than 6) maintains a stable amorphous state at hightemperatures not less than 800° C. when the value of m is less than 6.However, as the value of m increases, i.e., as the ratio of ZnO toInGaO₃ increases and the composition approaches to the ZnO composition,the composition tends to be more crystallizable. Thus, the value of m ispreferably less than 6 for the channel layer of the amorphous TFT. Adesired amorphous oxide can be obtained by adjusting the composition ofthe target material (e.g., a polycrystalline material) for deposition,such as sputtering deposition or pulsed laser deposition (PLD), tocomply with m<6.

In the amorphous oxide described above, Zn in the composition ratio ofInGaZn may be replaced by Zn_(1-x)Mg_(x). The possible amount of Mg forreplacement is within the range of 0<x≦1. When the replacement with Mgis conducted, the electron mobility of the oxide film decreases comparedto a film containing no Mg. However, the extent of decrease is small,and the electron carrier concentration can be decreased compared to whenno replacement is conducted. Thus, this is more preferable for thechannel layer of a TFT. The amount of Mg for replacement is preferablymore than 20% and less than 85% (0.2<x<0.85 in term of x) and morepreferably 0.5<x<0.85.

The amorphous oxide may be appropriately selected from In oxides,In_(x)Zn_(1-x) oxides (0.2≦x≦1), In_(x)Sn_(1-x) oxides (0.8≦x≦1), andIn_(x)(Zn, Sn)_(1-x) oxides (0.15≦x≦1). The ratio of Zn to Sn in theIn_(x)(Zn, Sn)_(1-x) oxides may be appropriately selected. Namely, anIn_(x)(Zn, Sn)_(1-x) oxide can be described asIn_(x)(Zn_(y)Sn_(1-y))_(1-x) oxide, and y is in the range of 1 to 0. Foran In oxide containing neither Zn nor Sn, In may be partly replaced byGa. In this case, the oxide can be described as an In_(x)Ga_(1-x) oxide(0≦x≦1).

(Method for Producing Amorphous Oxide)

The amorphous oxide used in the present invention can be prepared by avapor phase deposition technique under the conditions indicated in theindividual examples below. For example, in order to obtain an InGaZnamorphous oxide, deposition is conducted by a vapor phase method such asa sputtering (SP) method, a pulsed laser deposition (PLD) method, or anelectron beam deposition method while using a polycrystalline sinterrepresented by InGaO₃(ZnO)_(m) as the target. From the standpoint ofmass productivity, the sputtering method is most suitable.

During the formation of an In₂O₃ or In—Zn—O amorphous oxide film or thelike, oxygen radicals may be added to the atmosphere. Oxygen radicalsmay be added through an oxygen radical generator. When there is need toincrease the electron carrier concentration after the film formation,the film is heated in a reducing atmosphere to increase the electroncarrier concentration. The resulting amorphous oxide film with adifferent electron carrier concentration was analyzed to determine thedependency of the electron mobility on the electron carrierconcentration, and the electron mobility increased with the electroncarrier concentration.

(Substrate)

The substrate for forming the TFT of the present invention may be aglass substrate, a plastic substrate, a plastic film, or the like.Moreover, as described below in EXAMPLES, the amorphous oxide of thepresent invention can be formed into a film at room temperature. Thus, aTFT can be formed on a flexible material such as a PET film. Moreover,the above-mentioned amorphous oxide may be appropriately selected toprepare a TFT from a material that is transparent in visible light notless than 400 nm or infrared light.

(Gate Insulating Film)

The gate insulating film of the TFT of the present invention ispreferably a gate insulating film composed of Al₂O₃, Y₂O₃, HfO₂, or amixed crystal compound containing at least two of these compounds. Whenthere is a defect at the interface between the gate insulating thin filmand the channel layer thin film, the electron mobility decreases andhysteresis occurs in the transistor characteristics. Moreover, leakcurrent greatly differs according to the type of the gate insulatingfilm. Therefore, a gate insulating film suitable for the channel layermust be selected.

Use of an Al₂O₃ film can decrease the leak current. Use of an Y₂O₃ filmcan reduce the hysteresis. Use of a high dielectric constant HfO₂ filmcan increase the field effect mobility. By using a film composed of amixed crystal of these compounds, a TFT having small leak current andhysteresis and large field effect mobility can be produced. the processfor forming the gate insulating film and the process for forming thechannel layer can be conducted at room temperature; thus, a TFT of astaggered or inverted staggered structure can be formed.

(Transistor)

When a field effect transistor includes a channel layer composed of anamorphous oxide film having an electron carrier concentration of lessthan 10¹⁸/cm³, a source terminal, a drain terminal, and a gate terminaldisposed on the gate insulating film, the current between the source anddrain terminals can be adjusted to about 10⁻⁷ A when a voltage of about5V is applied between the source and drain terminals without applicationof a gate voltage. The theoretical lower limit of the electron carrierconcentration is 10⁵/cm³ or less assuming that the electrons in thevalence band are thermally excited. The actual possibility is that thelower limit is about 10¹²/cm³.

When Al₂O₃, Y₂O₃, or HfO₂ alone or a mixed crystal compound containingat least two of these compounds is used in the gate insulating layer,the leak voltage between the source gate terminals and the leak voltagebetween the drain and gate terminals can be adjusted to about 10⁻⁷ A,and a normally off transistor can be realized.

The electron mobility of the oxide crystals increases as the overlap ofthe s orbits of the metal ion increases. The oxide crystals of Zn, In,and Sn having large atomic numbers exhibit high electron mobility of 0.1to 200 cm²/(V·sec). Since ionic bonds are formed between oxygen andmetal ions in an oxide, electron mobility substantially comparable tothat in a crystallized state can be exhibited in an amorphous state inwhich there is no directionality of chemical bonding, the structure israndom, and the directions of the bonding are nonuniform. In contrast,by replacing Zn, In, and Sn each with an element having a smaller atomicnumber, the electron mobility can be decreased. Thus, by using theamorphous oxide described above, the electron mobility can be controlledwithin the range of about 0.01 cm²/(V·sec) to 20 cm²/(V·sec).

In a typical compound, the electron mobility decreases as the carrierconcentration increases due to the dispersion between the carriers. Incontrast, the amorphous oxide of the present invention exhibitsincreased electron mobility with the increasing electron carrierconcentration. The physical principle that lies behind this phenomenonis not clearly identified.

Once a voltage is applied to the gate terminal, electrons are injectedinto the amorphous oxide channel layer, and current flows between thesource and drain terminals, thereby allowing the part between the sourceand drain terminals to enter an ON state. According to the amorphousoxide film of the present invention, since the electron mobilityincreases with the electron carrier concentration, the current thatflows when the transistor is turned ON can be further increased. Inother words, the saturation current and the on/off ratio can be furtherincreased. When the amorphous oxide film having high electron mobilityis used as the channel layer of a TFT, the saturation current can beincreased and the switching rate of the TFT can be increased, therebyachieving high-speed operation.

For example, when the electron mobility is about 0.01 cm²/(V·sec), thematerial can be used in a channel layer of a TFT for driving a liquidcrystal display element. By using an amorphous oxide film having anelectron mobility of about 0.1 cm²/(V·sec), a TFT that has performancecomparable or superior to the TFT using an amorphous silicon film andthat can drive a display element for moving images can be produced.

In order to realize a TFT that requires large current, e.g., for drivinga current-driven organic light-emitting diode, the electron mobility ispreferably more than 1 cm²/(V·sec). Note than when the amorphous oxideof the present invention that exhibits degenerate conduction is used inthe channel layer, the current that flows at a high carrierconcentration, i.e., the saturation current of the transistor, showsdecreased dependency on temperature, and a TFT with superior temperaturecharacteristics can be realized.

EXAMPLES Example 1 Preparation of Amorphous In—Ga—Zn—O Thin Film by PLDMethod

A film was formed in a PLD device shown in FIG. 7. In the drawing,reference numeral 701 denotes a rotary pump (RP), 702 denotes a turbomolecular pump (TMP), 703 denotes a preparation chamber, 704 denotes enelectron gun for RHEED, 705 denotes a substrate holder for rotating andvertically moving the substrate, 706 denotes a laser entrance window,707 denotes a substrate, 708 denotes a target, 709 denotes a radicalsource, 710 denotes a gas inlet, 711 denotes a target holder forrotating and vertically moving the target, 712 denotes a by-pass line,713 denotes a main line, 714 denotes a turbo molecular pump (TMP), 715denotes a rotary pump (RP), 716 denotes a titanium getter pump, and 717denotes a shutter. In the drawing, 718 denotes ionization gauge (IG),719 denotes a Pirani gauge (PG), 720 denotes a Baratron gauge (BG), and721 denotes a deposition chamber.

An In—Ga—Zn—O amorphous oxide semiconductor thin film was formed on aSiO₂ glass substrate (# 1737 produced by Corning) by a pulsed laserdeposition method using a KrF excimer laser. As the pre-depositiontreatment, the substrate was degreased with ultrasonic waves in acetone,ethanol, and ultrapure water for 5 minutes each, and then dried in airat 100° C.

An InGaO₃(ZnO)₄ sinter target (size: 20 mm in dia., 5 mm in thickness)was used as the polycrystalline target. This target was prepared bywet-mixing the starting materials, In₂O₃:Ga₂O₃:ZnO (each being a 4Nreagent), in a solvent (ethanol), calcining (1000° C., 2 h) theresulting mixture, dry-milling the calcined mixture, and sintering theresulting mixture (1550° C., 2 h). The electrical conductivity of thetarget obtained was 90 (S/cm).

The ultimate vacuum of the deposition chamber was adjusted to 2×10⁻⁶(Pa), and the oxygen partial pressure during the deposition wascontrolled to 6.5 (Pa) to form a film. The oxygen partial pressureinside the chamber 721 was 6.5 Pa, and the substrate temperature was 25°C. The distance between the target 708 and the substrate 707 fordeposition was 30 (mm). The power of the KrF excimer laser entering fromthe entrance window 716 was in the range of 1.5 to 3 (mJ/cm²/pulse). Thepulse width was 20 (nsec), the repetition frequency was 10 (Hz), and thebeam spot diameter was 1×1 (mm square). A film was formed at adeposition rate of 7 (nm/min).

The resulting thin film was subjected to grazing incidence x-raydiffraction (thin film method, incident angle: 0.5°), but no cleardiffraction peak was observed. Thus, the In—Ga—Zn—O thin film obtainedwas assumed to be amorphous. The X-ray reflectance was determined, andthe pattern was analyzed. It was observed that the root mean squareroughness (Rrms) of the thin film was about 0.5 nm, and the filmthickness was about 120 nm. The results of the fluorescence X-ray showedthat the metal composition ratio of the thin film wasIn:Ga:Zn=0.98:1.02:4. The electrical conductivity was less than about10⁻² S/cm. The electron carrier concentration and the electron mobilitywere presumably about 10¹⁶/cm³ or less and about 5 cm²/(V·sec),respectively.

Based on the analysis of the optical absorption spectrum, the energywidth of the forbidden band of the amorphous thin film prepared wasdetermined to be about 3 eV. Based on these values, it was found thatthe In—Ga—Zn—O thin film had an amorphous phase close to the compositionof the crystals of InGaO₃(ZnO)₄, had fewer oxygen defects, and was aflat, transparent thin film with low electrical conductivity.

Specific description is now presented with reference to FIG. 1. FIG. 1shows a change in electron carrier concentration of the oxide formedinto a film against changes in oxygen partial pressure when anIn—Ga—Zn—O transparent amorphous oxide thin film represented byInGaO₃(ZnO)₄ in an assumed crystal state is formed under the sameconditions as in this EXAMPLE.

As shown in FIG. 1, the electron carrier concentration decreased to lessthan 10¹⁸/cm³ when the film was formed in an atmosphere at a high oxygenpartial pressure of more than 4.5 Pa under the same conditions as thisexample. In this case, the temperature of the substrate was maintainedsubstantially at room temperature without intentional heating. Thesubstrate temperature is preferably less than 100° C. when a flexibleplastic film is used as the substrate.

By further increasing the oxygen partial pressure, the electron carrierconcentration was further decreased. For example, as shown in FIG. 1,the number of the electron carriers of the InGaO₃(ZnO)₄ thin filmdeposited at a substrate temperature of 25° C. and an oxygen partialpressure of 5 Pa decreased to 10¹⁶/cm³.

The thin film obtained had an electron mobility exceeding 1 cm²/(V·sec),as shown in FIG. 2. However, according to the pulsed laser depositionmethod of the present invention, the surface of the film deposited willhave irregularities at an oxygen partial pressure of 6.5 Pa or more, andthus, the it is difficult to use the thin film as a channel layer of aTFT. Therefore, by using an In—Ga—Zn—O transparent amorphous oxide thinfilm having a composition of InGaO₃(ZnO)_(m) (m is less than 6) in acrystal state prepared by a pulsed laser deposition method in anatmosphere having an oxygen partial pressure exceeding 4.5 Pa,preferably exceeding 5 Pa, but less than 6.5 Pa, a normally offtransistor can be prepared.

The electron mobility of this thin film was more than 1 cm²/(V·sec), andthe on/off ratio thereof was increased to over 10³. As is describedabove, in forming an InGaZn oxide film by a PLD method under theconditions set forth in this example, the oxygen partial pressure ispreferably controlled to not less than 4.5 Pa but less than 6.5 Pa.Whether an electron carrier concentration of 10¹⁸/cm³ is realizeddepends on the conditions of the oxygen partial pressure, theconfiguration of the deposition device, the materials for deposition,the composition, and the like.

Example 2 Formation of Amorphous InGaO₃(ZnO) and InGaO₃(ZnO)₄ OxideFilms by PLD Method

In—Zn—Ga—O amorphous oxide films were deposited on glass substrates(#1737 produced by Corning) by using polycrystalline sinters representedby InGaO₃(ZnO) and InGaO₃(ZnO)₄ as the targets by a PLD method using KrFexcimer laser. The same PLD deposition device as shown in EXAMPLE 1 wasused, and the deposition was conducted under the same conditions. Thesubstrate temperature during the deposition was 25° C.

Each film obtained thereby was subjected to grazing incidence x-raydiffraction (thin film method, incident angle: 0.5°) for the filmsurface. No clear diffraction peak was detected. The In—Zn—Ga—O filmsprepared from the two targets were both amorphous.

The In—Zn—Ga—O amorphous oxide films on the glass substrates were eachanalyzed to determine the x-ray reflectance. Analysis of the patternfound that the root mean average roughness (Rrms) of the thin film wasabout 0.5 mm and that the thickness was about 120 nm. Fluorescence x-rayanalysis (XRF) showed that the ratio of the metal atoms of the filmobtained from the target composed of the polycrystalline sinterrepresented by InGaO₃(ZnO) was In:Ga:Zn=1.1:1.1:0.9 and that the ratioof the metal atoms of the film obtained from the target composed of thepolycrystalline sinter represented by InGaO₃(ZnO)₄ wasIn:Ga:Zn=0.98:1.02:4.

The electron carrier concentration of the amorphous oxide film obtainedfrom the target composed of the polycrystalline sinter represented byInGaO₃(ZnO)₄ was measured while changing the oxygen partial pressure ofthe atmosphere during the deposition. The results are shown in FIG. 1.By forming the film in the atmosphere having an oxygen partial pressureexceeding 4.5 Pa, the electron carrier concentration could be decreasedto less than 10¹⁸/cm³. In this case, the temperature of the substratewas maintained substantially at room temperature without intentionalheating. When the oxygen partial pressure was less than 6.5 Pa, thesurface of the amorphous oxide film obtained was flat.

When the oxygen partial pressure was 5 Pa, the electron carrierconcentration and the electrical conductivity of the amorphous oxidefilm obtained from the target composed of the polycrystalline sinterrepresented by InGaO₃(ZnO)₄ were 10¹⁶/cm³ and 10⁻² S/cm, respectively.The electron mobility was presumably about 5 cm²/(V·sec). Based on theanalysis of the optical absorption spectrum, the energy width of theforbidden band of the amorphous thin film prepared was determined to beabout 3 eV. The electron carrier concentration could be furtherdecreased as the oxygen partial pressure was increased from 5 Pa.

As shown in FIG. 1, the In—Zn—Ga—O amorphous oxide film deposited at asubstrate temperature of 25° C. and an oxygen partial pressure of 6 Paexhibited a decreased electron carrier concentration of 8×10¹⁵/cm³(electrical conductivity: about 8×10⁻³ S/cm). The resulting film wasassumed to have an electron mobility of more than 1 cm²/(V·sec).However, according to the PLD method, irregularities were formed in thesurface of the film deposited at an oxygen partial pressure of 6.5 Pa ormore, and thus it was difficult to use the film as the channel layer ofthe TFT.

The relationship between the electron carrier concentration and theelectron mobility of the In—Zn—Ga—O amorphous oxide film prepared fromthe target composed of the polycrystalline sinter represented byInGaO₃(ZnO)₄ at different oxygen partial pressures was investigated. Theresults are shown in Table 2. When the electron carrier concentrationincreased from 10¹⁶/cm³ to 10²⁰/cm³, the electron mobility increasedfrom about 3 cm²/(V·sec) to about 11 cm²/(V·sec). The same tendency wasobserved for the amorphous oxide film prepared from the target composedof the polycrystalline sinter represented by InGaO₃(ZnO).

An In—Zn—Ga—O amorphous oxide film formed on a polyethyleneterephthalate (PET) film having a thickness of 200 μm instead of theglass substrate also showed similar characteristics.

Example 3 Formation of In—Zn—Ga—O Amorphous Oxide Film by SP Method

Formation of a film by a high-frequency SP method using argon gas as theatmosphere gas is described. The SP method was conducted using thedevice shown in FIG. 8. In the drawing, reference numeral 807 denotes asubstrate for deposition, 808 denotes a target, 805 denotes a substrateholder equipped with a cooling mechanism, 814 denotes a turbo molecularpump, 815 denotes a rotary pump, 817 denotes a shutter, 818 denotes anionization gauge, 819 denotes a Pirani gauge, 821 denotes a depositionchamber, and 830 denotes a gate valve. A SiO₂ glass substrate (#1737produced by Corning) was used as the substrate 807 for deposition. Asthe pre-deposition treatment, the substrate was degreased withultrasonic waves in acetone, ethanol, and ultrapure water for 5 minuteseach, and then dried in air at 100° C.

An InGaO₃(ZnO)₄ polycrystalline sinter (size: 20 mm in dia., 5 mm inthickness) was used as the target material. The sinter was prepared bywet-mixing the starting materials, In₂O₃:Ga₂O₃:ZnO (each being a 4Nreagent), in a solvent (ethanol), calcining (1000° C., 2 h) theresulting mixture, dry-milling the calcined mixture, and sintering theresulting mixture (1550° C., 2 h). The target 808 had an electricalconductivity of 90 (S/cm) and was in a semi-insulating state.

The ultimate vacuum inside the deposition chamber 821 was 1×10⁻⁴ (Pa).The total pressure of the oxygen gas and the argon gas during thedeposition was controlled at a predetermined value within the range of 4to 0.1×10⁻¹ (Pa), and the oxygen partial pressure was changed in therange of 10⁻³ to 2×10⁻¹ (Pa) by changing the partial pressure ratio ofthe argon gas and oxygen. The substrate temperature was roomtemperature, and the distance between the target 808 and the substrate807 for deposition was 30 (mm). The current injected was RF 180 W, andthe deposition rate was 10 (nm/min).

The resulting film was subjected to grazing incidence x-ray diffraction(thin film method, incident angle=0.5°) for the film surface, but noclear diffraction peak was observed. Thus, the In—Zn—Ga—O thin filmobtained was proved to be amorphous. The X-ray reflectance wasdetermined, and the pattern was analyzed. It was observed that the rootmean square roughness (Rrms) of the thin film was about 0.5 nm, and thefilm thickness was about 120 nm. The results of the fluorescence X-rayshowed that the metal composition ratio of the thin film wasIn:Ga:Zn=0.98:1.02:4.

The electrical conductivity of the amorphous oxide film obtained bychanging the oxygen partial pressure in the atmosphere during thedeposition was measured. The results are shown in FIG. 3. As shown inFIG. 3, the electrical conductivity could be decreased to less than 10S/cm by forming the film in an atmosphere at a high oxygen partialpressure exceeding 3×10⁻² Pa.

By further increasing the oxygen partial pressure, the number ofelectron carriers could be decreased. For example, as shown in FIG. 3,the electrical conductivity of an InGaO₃(ZnO)₄ thin film deposited at asubstrate temperature of 25° C. and an oxygen partial pressure of 10⁻¹Pa was decreased to about 10⁻¹⁰ S/cm. An InGaO₃(ZnO)₄ thin filmdeposited at an oxygen partial pressure exceeding 10⁻¹ Pa hadexcessively high electrical resistance and thus the electricalconductivity thereof could not be measured. However, extrapolation wasconducted for the value observed from a film having a high electroncarrier concentration, and the electron mobility was assumed to be about1 cm²/(V·sec).

In short, a normally off transistor having an on/off ratio exceeding 10³could be made by using a transparent amorphous oxide thin film which wascomposed of In—Ga—Zn—O prepared by a sputter deposition method in argongas atmosphere at an oxygen partial pressure more than 3×10⁻² Pa,preferably more than 5×10⁻¹ Pa, and which was represented byInGaO₃(ZnO)₄ (m is a natural number less than 6) in a crystallizedstate.

When the device and starting materials set forth in this example areused, the oxygen partial pressure during the sputter deposition is, forexample, in the range of 3×10⁻² Pa to 5×10⁻¹ Pa. The electron mobilityof the thin films prepared by the pulsed laser deposition method and thesputtering method increases with the number of the conduction electrons,as shown in FIG. 2.

As described above, by controlling the oxygen partial pressure, oxygendefects can be reduced, and therefore the electron carrier concentrationcan be reduced. Unlike in the polycrystalline state, in the amorphousstate, there is essentially no grain interface; therefore, an amorphousthin film with high electron mobility can be obtained. Note that when apolyethylene terephthalate (PET) film having a thickness of 200 μm wasused instead of the glass substrate, the resulting InGaO₃(ZnO)₄amorphous oxide thin film exhibited similar characteristics.

Example 4 Formation of In—Zn—Ga—Mg—O Amorphous Oxide Film by PLD Method

Formation of an InGaO₃(Zn_(1-x)Mg_(x)O)₄ film (0<x<1) on a glasssubstrate by a PLD method is described. The same deposition device shownin FIG. 7 was used as the deposition device. A SiO₂ glass substrate(#1737 produced by Corning) was prepared as the substrate fordeposition. As the pre-deposition treatment, the substrate was degreasedwith ultrasonic waves in acetone, ethanol, and ultrapure water for 5minutes each, and then dried in air at 100° C.

An InGa (Zn_(1-x)Mg_(x)O)₄ (0<x<1) sinter (size: 20 mm in dia., 5 mm inthickness) was used as the target. The target was prepared by wet-mixingthe starting materials, In₂O₃:Ga₂O₃:ZnO:MgO (each being a 4N reagent),in a solvent (ethanol), calcining (1000° C., 2 h) the resulting mixture,dry-milling the calcined mixture, and sintering the resulting mixture(1550° C., 2 h).

The ultimate vacuum inside the deposition chamber was 2×10⁻⁶ (Pa), andthe oxygen partial pressure during the deposition was 0.8 (Pa). Thesubstrate temperature was room temperature (25° C.), and the distancebetween the target and the substrate for deposition was 30 (mm). Thepower of the KrF excimer laser was 1.5 (mJ/cm²/pulse), the pulse widthwas 20 (nsec), the repetition frequency was 10 (Hz), and the beam spotdiameter was 1×1 (mm square). The deposition rate was 7 (nm/min).

The resulting film was subjected to grazing incidence x-ray diffraction(thin film method, incident angle: 0.5°) for the film surface, but noclear diffraction peak was observed. Thus, the In—Zn—Ga—Mg—O thin filmobtained was proved to be amorphous. The surface of the resulting filmwas flat.

The dependency on the value x of the electrical conductivity, electroncarrier concentration, and electron mobility of In—Zn—Ga—Mg—O amorphousoxide films deposited in atmosphere at an oxygen partial pressure of 0.8Pa was investigated by using targets of different x values. Note that ahigh-resistance amorphous InGaO₃(Zn_(1-x)Mg_(x)O)_(m) film could beobtained at an oxygen partial pressure of less than 1 Pa as long as thepolycrystalline InGaO₃(Zn_(1-x)Mg_(x)O)_(m) (m is a natural number lessthan 6; 0<x≦1) was used as the target.

The results are shown in FIG. 4. The results showed that the electroncarrier concentration of an amorphous oxide film deposited by a PLDmethod in an atmosphere at an oxygen partial pressure of 0.8 Pa could bereduced to less than 10¹⁸/cm³ when the value x was more than 0.4. Theelectron mobility of the amorphous oxide film with x exceeding 0.4 wasmore than 1 cm²/(V·sec). As shown in FIG. 4, when a target in which Znwas substituted with 80 at % Mg was used, the electron carrierconcentration of the film obtained by the pulsed laser deposition methodin an atmosphere at an oxygen partial pressure of 0.8 Pa could bereduced to less than 10¹⁶/cm³.

Although the electron mobility of these films is low compared to that ofMg-free films, the degree of decrease is small, while the electronmobility at room temperature is about 5 cm²/(V·sec), i.e., higher thanthat of amorphous silicon by one order of magnitude. When deposition isconducted under the same conditions, the electrical conductivity and theelectron mobility both decrease with an increase in Mg content. Thus,the Mg content is preferably more than 20 at % but less than 85 at %(0.2<x<0.85 in terms of x), and more preferably 0.5<x<0.85.

An InGaO₃(Zn_(1-x)Mg_(x)O)₄ (0<x<1) amorphous oxide film formed on apolyethylene terephthalate (PET) film having a thickness of 200 μminstead of the glass substrate also showed similar characteristics.

Example 5 Formation of In₂O₃ Amorphous Oxide Film by PLD

Formation of an indium oxide film is now described. The depositiondevice shown in FIG. 7 was used as the deposition device. A SiO₂ glasssubstrate (#1737 produced by Corning) was prepared as the substrate fordeposition. As the pre-deposition treatment, the substrate was degreasedwith ultrasonic waves in acetone, ethanol, and ultrapure water for 5minutes each, and then dried in air at 100° C.

An In₂O₃ sinter (size: 20 mm in dia., 5 mm in thickness) was used as thetarget. The target was prepared by calcining the starting material In₂O₃(a 4N reagent) (1000° C., 2 h), dry milling the calcined material, andsintering the resulting material (1550° C., 2 h).

The ultimate vacuum inside the deposition chamber was 2×10⁻⁶ (Pa), andthe oxygen partial pressure during the deposition was 5 (Pa). The steampartial pressure was 0.1 (Pa), and 200 W was applied to the oxygenradical generator to produce oxygen radicals. The substrate temperaturewas room temperature. The distance between the target and the substratefor deposition was 40 (mm). The power of the KrF excimer laser was 0.5(mJ/cm²/pulse), the pulse width was 20 (nsec), the repetition frequencywas 10 (Hz), and the beam spot diameter was 1×1 (mm square). Thedeposition rate was 3 (nm/min).

The resulting film was subjected to grazing incidence x-ray diffraction(thin film method, incident angle: 0.5°) for the film surface, but noclear diffraction peak was observed. Thus, the In—O thin film obtainedwas proved to be amorphous. The film thickness was 80 nm. The electroncarrier concentration and the electron mobility of the In—O amorphousoxide film obtained were 5×10¹⁷/cm³ and about 7 cm²/(V·sec),respectively.

Example 6 Formation of In—Sn—O Amorphous Oxide Film by PLD

Deposition of an In—Sn—O amorphous oxide film having a thickness of 200μm by a PLD method is described. A SiO₂ glass substrate (#1737 producedby Corning) was prepared as the substrate for deposition. As thepre-deposition treatment, the substrate was degreased with ultrasonicwaves in acetone, ethanol, and ultrapure water for 5 minutes each, andthen dried in air at 100° C.

An In₂O₃—SnO₂ sinter (size: 20 mm in dia., 5 mm in thickness) wasprepared as the target by wet-mixing the starting materials, In₂O₃—SnO₂(a 4N reagent), in a solvent (ethanol), calcining the resulting mixture(1000° C., 2 h), dry milling the calcined mixture, and sintering theresulting mixture (1550° C., 2 h). The composition of the target was(In_(0.9)Sn_(0.1))₂O_(3.1) polycrystal.

The ultimate vacuum inside the deposition chamber was 2×10⁻⁶ (Pa), theoxygen partial pressure during the deposition was 5 (Pa), and thenitrogen partial pressure was 0.1 (Pa). Then 200 W is applied to theoxygen radical generator to produce oxygen radicals. The substratetemperature during the deposition was room temperature. The distancebetween the target and the substrate for deposition was 30 (mm). Thepower of the KrF excimer laser was 1.5 (mJ/cm²/pulse), the pulse widthwas 20 (nsec), the repetition frequency was 10 (Hz), and the beam spotdiameter was 1×1 (mm square).

The deposition rate was 6 (nm/min). The resulting film was subjected tograzing incidence x-ray diffraction (thin film method, incident angle:0.5°) for the film surface, but no clear diffraction peak was observed.Thus, the In—Sn—O thin film obtained was proved to be amorphous. Theelectron carrier concentration and the electron mobility of the In—Sn—Oamorphous oxide film obtained were 8×10¹⁷/cm³ and about 5 cm²/(V·sec),respectively. The film thickness was 100 nm.

Example 7 Formation of In—Ga—O Amorphous Oxide Film by PLD Method

Deposition of an indium gallium oxide is described next. A SiO₂ glasssubstrate (#1737 produced by Corning) was prepared as the substrate fordeposition. As the pre-deposition treatment, the substrate was degreasedwith ultrasonic waves in acetone, ethanol, and ultrapure water for 5minutes each, and then dried in air at 100° C.

A (In₂O₃)_(1-x)—(Ga₂O₃)_(x) (x=0 to 1) sinter was prepared as the target(size: 20 mm in dia., 5 mm in thickness). For example, when x=0.1, thetarget was an (In_(0.9)Ga_(0.1))₂O₃ polycrystalline sinter. This targetwas obtained by wet-mixing the starting materials, In₂O₃—Ga₂O₃ (4Nreagent), in a solvent (ethanol), calcining the resulting mixture (1000°C., 2 h), dry-milling the calcined mixture, and sintering the resultingmixture (1550° C., 2 h).

The ultimate vacuum inside the deposition chamber was 2×10⁻⁶ (Pa), andthe oxygen partial pressure during the deposition was 1 (Pa). Thesubstrate temperature during the deposition was room temperature. Thedistance between the target and the substrate for deposition was 30(mm). The power of the KrF excimer laser was 1.5 (mJ/cm²/pulse), thepulse width was 20 (nsec), the repetition frequency was 10 (Hz), and thebeam spot diameter was 1×1 (mm square). The deposition rate was 6(nm/min).

The resulting film was subjected to grazing incidence x-ray diffraction(thin film method, incident angle: 0.5°) for the film surface, but noclear diffraction peak was observed. Thus, the In—Ga—O thin filmobtained was proved to be amorphous. The film thickness was 120 nm. Theelectron carrier concentration and the electron mobility of the In—Ga—Oamorphous oxide film obtained were 8×10¹⁶/cm³ and about 1 cm²/(V·sec),respectively.

Example 8 Preparation of TFT Element (Glass Substrate) Using In—Zn—Ga—OAmorphous Oxide Film

A top-gate TFT element shown in FIG. 5 was prepared. First, anIn—Zn—Ga—O amorphous film 120 nm in thickness for use as a channel layer(2) was formed on a glass substrate (1) by a method of preparing theIn—Ga—Zn—O amorphous oxide film according to EXAMPLE 1 at an oxygenpartial pressure of 5 Pa while using a polycrystalline sinterrepresented by InGaO₃(ZnO)₄ as the target.

An In—Ga—Zn—O amorphous film having high electrical conductivity and agold film each 30 nm in thickness were deposited on the In—Ga—Zn—Oamorphous film by a PLD method while controlling the oxygen partialpressure inside the chamber to less than 1 Pa, and a drain terminal (5)and a source terminal (6) were formed by a photolithographic method anda lift-off method.

Lastly, an Y₂O₃ film (thickness: 90 nm, relative dielectric constant:about 15, leak current density: 10⁻³ A/cm² upon application of 0.5MV/cm) for use as a gate insulating film (3) was formed by an electronbeam deposition method, and gold was deposited on the Y₂O₃ film. A gateterminal (4) was formed by a photolithographic method and a lift-offmethod. The channel length was 50 μm and the channel width was 200 μm.

(Evaluation of Characteristics of TFT Element)

FIG. 6 shows the current-voltage characteristic of the TFT elementmeasured at room temperature. Since the drain current I_(DS) increasedwith the drain voltage V_(DS), the channel was found to be of ann-conductivity type. This is consistent with the fact that the amorphousIn—Ga—Zn—O oxide film is an n-type conductor. I_(DS) was saturated(pinch-off) at about V_(DS)=6 V, which was a typical behavior forsemiconductor transistors. The gain characteristic was determined, andthe threshold value of the gate voltage V_(GS) when V_(DS)=4 V wasapplied was about −0.5 V. Upon application of V_(GS)=6 V and V_(DS)=10V, current of I_(DS)=1.0×10⁻⁵ A flowed. This is because carriers wereinduced in the In—Ga—Zn—O amorphous semiconductor thin film, i.e., aninsulator, due to the gate bias. The on/off ratio of the transistorexceeded 10³. The field effect mobility was determined from the outputcharacteristics. As a result, a field effect mobility of about 7cm²(Vs)⁻¹ was obtained in the saturation region.

The same measurements were carried out on the element while irradiatingthe element with visible light, but no change in transistorcharacteristics was observed. Note that the film can be used as achannel layer of a TFT by controlling the electron carrier concentrationof the amorphous oxide to less than 10¹⁸/cm³. An electron carrierconcentration of 10¹⁷/cm³ or less was more preferable, and an electroncarrier density of 10¹⁶/cm³ or less was yet more preferable.

According to this example, a tin film transistor having a channel layerwith an increased electron carrier concentration, a high electricalresistance, and high electron mobility can be realized. The amorphousoxide described above exhibited excellent characteristics such asincreased electron mobility with increasing electron carrierconcentration and degenerate conduction.

In this example, the thin film transistor is formed on a glasssubstrate; however, since the film formation can be conducted at roomtemperature, a substrate such as a plastic board or a film can be used.Moreover, the amorphous oxide obtained in this example hardly absorbsvisible light, and thus a transparent flexible TFT can be realized.

Example 9 Preparation of TFT Element Using In—Zn—Ga—O Amorphous OxideFilm

A top-gate TFT element shown in FIG. 5 was prepared. In particular, anIn—Zn—Ga—O amorphous oxide film 120 nm in thickness for use as a channellayer (2) was formed on a polyethylene terephthalate (PET) film (1) by adeposition method of EXAMPLE 2 in an atmosphere at an oxygen partialpressure of 5 Pa using a polycrystalline sinter represented byInGaO₃(ZnO) as the target.

An In—Zn—Ga—O amorphous oxide film having high electrical conductivityand a gold film each 30 nm in thickness were deposited on the In—Zn—Ga—Oamorphous oxide film by the PLD method at an oxygen partial pressureinside the chamber of less than 1 Pa, and a drain terminal (5) and asource terminal (6) were formed by a photolithographic method and alift-off method.

Lastly, a gate insulating film (3) was formed by an electron beamdeposition method and gold is deposited thereon. A gate terminal (4) wasthen formed by a photolithographic method and a lift-off method. Thechannel length was 50 μm and the channel width was 200 μm. Three typesof TFTs with the above-described structure were prepared using Y₂O₃(thickness: 140 nm), Al₂O₃ (thickness: 130 nm) and HfO₂ (thickness: 140nm), respectively.

(Evaluation of Characteristics of TFT Element)

The current-voltage characteristic of the TFT element measured at roomtemperature was similar to one shown in FIG. 6. Namely, since the draincurrent I_(DS) increased with the drain voltage V_(DS), the channel wasfound to be of an n-conductivity type. This is consistent with the factthat the amorphous In—Ga—Zn—O amorphous oxide film is an n-typeconductor. I_(DS) was saturated (pinch-off) at V_(DS)=about 6 V, whichwas a typical behavior for semiconductor transistors. When V_(GS)=6 Vand V_(DS)=10 V, current of I_(ds)=1×10⁻⁵ A flowed. This is becausecarriers were induced in the In—Ga—Zn—O amorphous oxide thin film, i.e.,an insulator, due to the gate bias. The on/off ratio of the transistorexceeded 10³. The field effect mobility was determined from the outputcharacteristics. As a result, a field effect mobility of about 7cm²(Vs)⁻¹ was obtained in the saturation region.

The element formed on the PET film was inflected at a radius ofcurvature of 30 mm, and the same transistor characteristic was measured.No change in transistor characteristic was observed.

The TFT including the gate insulating film made from the Al₂O₃ film alsoshowed similar transistor characteristics to those shown in FIG. 6. WhenV_(GS)=6 V and V_(DS)=0, current of I_(ds)=10⁻⁸ A flowed, and whenV_(DS)=10 V, current of I_(ds)=5.0×10⁻⁶ A flowed. The on/off ratio ofthe transistor exceeded 10². The field effect mobility was determinedfrom the output characteristics. As a result, a field effect mobility ofabout 2 cm²(Vs)⁻¹ was obtained in the saturation region.

The TFT including the gate insulating film made from the HfO₂ film alsoshowed similar transistor characteristics to those shown in FIG. 6. WhenV_(g)=0 V, current of I_(ds)=10⁻⁸ A flowed, and when V_(g)=10 V, currentof I_(ds)=1.0×10⁻⁶ A flowed. The on/off ratio of the transistor exceeded10². The field effect mobility was determined from the outputcharacteristics. As a result, a field effect mobility of about 10cm²(Vs)⁻¹ was obtained in the saturation region.

Example 10 Preparation of TFT Element Using In₂O₃ Amorphous Oxide Filmby PLD Method

A top-gate TFT element shown in FIG. 5 was prepared. First, an In₂O₃amorphous oxide film 80 nm in thickness for use as a channel layer (2)was formed on a polyethylene terephthalate (PET) film (1) by thedeposition method of EXAMPLE 5.

An In₂O₃ amorphous oxide film having high electrical conductivity and agold layer each 30 nm in thickness were formed on this In₂O₃ amorphousoxide film by the PLD method at an oxygen partial pressure inside thechamber of less than 1 Pa while applying zero voltage to the oxygenradical generator. A drain terminal (5) and a source terminal (6) werethen formed by a photolithographic method and a lift-off method.

Lastly, an Y₂O₃ film for use as a gate insulating film (3) was formed byan electron beam deposition method, and gold was deposited on the Y₂O₃film. A gate terminal (4) was formed by a photolithographic method and alift-off method.

(Evaluation of Characteristics of TFT Element)

The current-voltage characteristics of the TFT element formed on the PETfilm were measured at room temperature. Since the drain current I_(DS)increased with the drain voltage V_(DS), the channel was found to be ofan n-conductivity type. This is consistent with the fact that theamorphous In—O amorphous oxide film is an n-type conductor. I_(DS) wassaturated (pinch-off) at V_(DS) about 5 V, which was a typical behaviorfor semiconductor transistors. When V_(GS)=6 V and V_(DS)=0, current ofI_(ds)=2×10⁻⁸ A flowed, and when V_(DS)=10 V current of I_(ds)=2.0×10⁻⁶A flowed. This is because carriers were induced in the In—O amorphousoxide thin film, i.e., an insulator, due to the gate bias. The on/offratio of the transistor was about 10². The field effect mobility wasdetermined from the output characteristics. As a result, a field effectmobility of about 10 cm²(Vs)⁻¹ was obtained in the saturation region.

The TFT element formed on a glass substrate showed similarcharacteristics. The element formed on the PET film was inflected at aradius of curvature of 30 mm, and the same transistor characteristicswere measured. No change in transistor characteristics was observed.

Example 11 Preparation of TFT Element Using In—Sn—O Amorphous Oxide Filmby PLD Method

A top gate TFT element shown in FIG. 5 was prepared. In particular, anIn—Sn—O amorphous oxide film 100 nm in thickness for use as a channellayer (2) was formed on a polyethylene terephthalate (PET) film (1) by adeposition method of EXAMPLE 6.

An In—Sn—O amorphous oxide film having high electrical conductivity anda gold film each 30 nm in thickness were deposited on this In—Sn—Oamorphous oxide film by the PLD method at an oxygen partial pressureinside the chamber of less than 1 Pa while applying zero voltage to theoxygen radical generator. A drain terminal (5) and a source terminal (6)were formed by a photolithographic method and a lift-off method.

Lastly, an Y₂O₃ film for use as a gate insulating film (3) was formed byan electron beam deposition method and gold was deposited thereon. Agate terminal (4) was then formed by a photolithographic method and alift-off method.

(Evaluation of Characteristics of TFT Element)

The current-voltage characteristic of the TFT element formed on the PETfilm was measured at room temperature. Since the drain current I_(DS)increased with the drain voltage V_(DS), the channel was found to be ofan n-conductivity type. This is consistent with the fact that theamorphous In—Sn—O amorphous oxide film is an n-type conductor. I_(DS)was saturated (pinch-off) at V_(DS)=about 6 V, which was a typicalbehavior for semiconductor transistors. When V_(GS)=6 V and V_(DS)=0 V,current of I_(ds)=5×10⁻⁸ A flowed, and when V_(DS)=10 V, current ofI_(ds)=5.0×10⁻⁵ A flowed. This is because carriers were induced in theIn—Sn—O amorphous oxide thin film, i.e., an insulator, due to the gatebias. The on/off ratio of the transistor was about 10³. The field effectmobility was determined from the output characteristics. As a result, afield effect mobility of about 5 cm²(Vs)⁻¹ was obtained in thesaturation region.

The TFT element formed on a glass substrate showed similarcharacteristics. The element formed on the PET film was inflected at aradius of curvature of 30 mm, and the same transistor characteristicswere measured. No change in transistor characteristics was observed.

Example 12 Preparation of TFT Element Using In—Ga—O Amorphous Oxide Filmby PLD Method

A top gate TFT element shown in FIG. 5 was prepared. In particular, anIn—Ga—O amorphous oxide film 120 nm in thickness for use as a channellayer (2) was formed on a polyethylene terephthalate (PET) film (1) bythe deposition method of EXAMPLE 7.

An In—Ga—O amorphous oxide film having high electrical conductivity anda gold film each 30 nm in thickness were formed on this In—Ga—Oamorphous oxide film by the PLD method at an oxygen partial pressureinside the chamber of less than 1 Pa while applying zero voltage to theoxygen radical generator. A drain terminal (5) and a source terminal (6)were formed by a photolithographic method and a lift-off method.

Lastly, an Y₂O₃ film for use as a gate insulating film (3) was formed byan electron beam deposition method and gold was deposited thereon. Agate terminal (4) was then formed by a photolithographic method and alift-off method.

(Evaluation of Characteristics of TFT Element)

The current-voltage characteristic of the TFT element formed on the PETfilm was measured at room temperature. Since the drain current I_(DS)increased with the drain voltage V_(DS), the channel was found to be ofan n-conductivity type. This is consistent with the fact that theamorphous In—Ga—O amorphous oxide film is an n-type conductor. I_(DS)was saturated (pinch-off) at V_(DS)=about 6 V, which was a typicalbehavior for semiconductor transistors. When V_(GS)=6V and V_(DS)=0 V,current of I_(ds)=1×10⁻⁸ A flowed, and when V_(DS)=10 V, current ofI_(ds)=1.0×10⁻⁶ A flowed. This corresponds to the induction of electroncarriers inside the insulator, In—Ga—O amorphous oxide film by the gatebias. The on/off ratio of the transistor was about 10². The field effectmobility was determined from the output characteristics. As a result, afield effect mobility of about 0.8 cm²(Vs)⁻¹ was obtained in thesaturation region.

The TFT element formed on a glass substrate showed similarcharacteristics. The element formed on the PET film was inflected at aradius of curvature of 30 mm, and the same transistor characteristicswere measured. No change in transistor characteristics was observed.

It should be noted that, as described in EXAMPLES above, the film can beused as a channel layer of a TFT by controlling the electron carrierconcentration to less than 10¹⁸/cm³. The electron carrier concentrationis more preferably 10¹⁷/cm³ or less and yet more preferably 10¹⁶/cm³ orless.

INDUSTRIAL APPLICABILITY

The amorphous oxide of the present invention can be used insemiconductor devices such as thin film transistors. The thin filmtransistors can be used as switching elements of LCDs and organic ELdisplays and are also widely applicable to see-through-type displays, ICcards, ID tags, etc.

1. A method of forming a thin film transistor device, comprising:providing a drain electrode; providing a source electrode; sputterdepositing a channel layer contacting the drain electrode and the sourceelectrode and including indium-gallium-zinc oxide having a transparent,semi-insulating, and amorphous state of a composition equivalent to theInGaO₃(ZnO)_(m) (wherein m is a natural number less than 6) in acrystallized state, wherein the sputtering deposition is performed in anatmosphere containing oxygen substantially at room temperature withoutadding impurity ions to increase electrical resistance, and at an oxygenpartial pressure to adjust an electron carrier concentration of 10¹³/cm³to 10¹⁷/cm³ as measured by Hall-effect measurement and an electronmobility of 1 cm²/(V·sec) or more at room temperature; providing a gateelectrode; and providing a gate insulating film positioned between thegate electrode and the channel.
 2. The method of forming a thin filmtransistor according to claim 1, wherein the substrate is one of a glassplate, a plastic plate or a plastic film. method and the oxygen partialpressure of over 3×10⁻² Pa.